A photoplethysmogram (“PPG”) is an optically obtained volumetric measurement of an organ (an optical plethysmogram). Photoplethysmography can be used in wearable activity monitors, medical equipment or other systems to optically detect blood volume changes in blood vessels to monitor blood flow, blood content, respiration rate and other circulatory conditions, where the intensity of back scattered light correlates to the amount of blood volume. PPG signals can be obtained in a number of different ways, including assessing absorption of light transmitted through, or reflected from, a patient's skin. A light source at a particular wavelength (typically, red, infrared or green) directs light toward the patient's skin. A photodiode or other optical sensor generates the PPG signal indicating the measured light transmission or reflection, and changes in the PPG signal can be used to detect the pulse rate of the patient's heart. PPG based heart rate estimation during motion is difficult, as motion artifacts show up in the PPG signal. The motion artifacts are caused by hemodynamic effects, tissue deformation, and sensor movement relative to the skin. Motion compensation techniques have been proposed to remove the motion component in the PPG signal using information from an external sensor reference, such as an accelerometer. Some approaches use spectrum subtraction to first remove the spectrum of the acceleration data from that of the PPG signal prior to heart rate estimation. Another motion compensation approach uses compressed sensing techniques combined with signal decomposition for de-noising and spectral tracking. The PPG signal fidelity can be further improved using normalized least mean squares (NLMS) and non-coherent combination in the frequency domain. A patient heart rate can be estimated from PPG signals using time-domain analysis, for example, band-pass filtering with a zero crossing detection, but this approach is largely unsuitable in the presence of motion artifacts. Frequency domain analysis generally involves choosing a highest peak in a frequency spectrum as the estimated patient heart rate. However, the PPG frequency spectrum typically includes multiple peaks within a range of realistic patient heart rates, and the dominant frequency domain peak does not always correspond to the actual patient heart rate, particularly in the presence of motion artifacts.